preview an edition
click Edit Details
scroll to edition number
change the number
add a note "entered incorrectly as edition 4 - now edition 5" or whatever
If you need to edit an edition number, simply open the artwork preview and click Edit Details.
In the Edition section simply update the number by clicking into the field.
Press Save and you're done!
Don't forget if you are swapping two edition numbers around to edit both edition numbers!